1. Field of the Invention
The present invention relates to a hydraulic brake system for vehicles, and more particularly to a hydraulic brake system for vehicles which is provided with a slip preventing apparatus.
2. Description of the Prior Art
A hydraulic brake control system is utilized for supplying the brake fluid pressure in response to the actuation of a brake pedal as an output. A master cylinder, namely a static hydraulic control apparatus, supplies the brake fluid pressure in response to the brake pedal as an output by the input of the brake fluid of the reservoir and a dynamic fluid pressure control apparatus supplies the brake fluid pressure regulated in response to the brake pedal as an output by the input of a power fluid pressure and which is provided with a power fluid pressure source.
A hydraulic booster has been proposed as the dynamic fluid pressure control apparatus, namely, a booster apparatus which can boost the master cylinder in response to the brake pedal as a boosting source of the power fluid pressure, so that the brake fluid pressure within the hydraulic booster is directly supplied to the wheel cylinder of the wheel. A slip preventing apparatus of the wheel, i.e., an antiskid apparatus, is provided and it is proposed to use the boost fluid pressure as a control pressure. This is disclosed, for example, in Japanese Patent Publication No. 56(1981)- 10219.
As described in Japanese Patent Laid Open Publication No. 60(1985)-38243, a boost fluid pressure is applied upon normal braking actuation and the stroke of the brake pedal is reduced. When a slip preventing apparatus is actuated, a boost fluid pressure circuit is blocked and brake fluid pressure within a wheel cylinder is decreased and increased by a valve apparatus so as to prevent lock of the wheel (hereinafter, anti-lock function).
If a braking force is supplied to a driven wheel, independent of the actuation of the brake pedal, in order to prevent the slip of the driven wheel upon the start or acceleration of the vehicles and the boost fluid pressure to the slip preventing apparatus is blocked so as to maintain a suitable rotational state of the driven wheel and a power fluid pressure source is directly communicated with the wheel cylinder of the driven wheel, an anti-slip function is provided. This function means the slip preventing apparatus from the point of view of the slip prevention of the wheel. That is to say, the brake force is automatically supplied to the driven wheel independent of the actuation of the brake pedal so that it is different from the slip preventing apparatus of the braking during the anti-lock function. However, it does not cooperate with the actuation of the brake pedal, so that the boost fluid pressure is not generated and a new independent brake fluid pressure source is required. The fluid pressure control can utilize the valve apparatus of the conventional slip preventing apparatus, so that the power fluid pressure source (auxiliary pressure source) is fluidically communicated wit the wheel cylinder of the driven wheel by a changeover valve. An example of such a simple construction is shown by Japanese Patent Laid Open Publication No. 60(1985)-38243.
Accordingly, the boost fluid pressure of the hydraulic booster is supplied to the wheel cylinder by the master cylinder upon the anti-lock control and the power fluid pressure is supplied to the wheel cylinder upon the anti-slip control by the above-mentioned changeover valve in the above mentioned prior art. Accordingly, a situation may arise where the power fluid pressure is supplied to the master cylinder at a transitional state of the changing and the reaction force acts on the brake pedal. This can not be avoided for the construction of the changeover valve, for example, of the type shown in FIG. 4. Therein, valve members 103, 104 mounted on both ends of the sliding direction of a plunger 102, slidably moved within a housing 101, are seated on one of seat members 105, 106 oppositely provided to the valve members 103, 104. In an electromagnetic valve 100 constructed as a changeover valve by the energization or deenergization of a solenoid 107, the communication between ports 108, 109 and ports 108, 110 is selectively attained. It may be happened that the valve members 103, 104 are separated from the seat members 105, 106 during transition of the changeover. However, as the anti-slip control is performed independent of the actuation of the brake pedal, the foregoing situation should not be permitted to occur. When the electromagnetic valve used as a changeover valve has a small energization requirement due to the miniaturization of the electromagnetic valve, there is the possibility for a delay of the response. In order to prevent the problem, a large number of open-close valves are used and the open-close state is maintained during the change, however, as the number of valves apparatus is increased the control system will be unduly complicated.